1. Field of Invention
The present invention relates to chemically prepared toner compositions containing hyper-branched and/or dendritic shaped polyester resins.
2. Description of the Related Art
Toners used in electrophotographic printers are of two primary types, namely chemically prepared toners (CPT) and conventional mechanically milled toners. Typically, CPT has significant advantage over mechanically milled toner, including better print quality, higher transfer efficiency and lower torque properties. Furthermore, it is much easier and more energy efficient to control toner particle size and shape compared to milled toners.
There are several types of CPTs, including Suspension Polymerization Toner (SPT), Emulsion Aggregation Toner (EAT or LAT), toner made from a Dispersion of Pre-formed Polymer in solvent (DPPT) and a type of CPT made from a “chemical milling” method. Of all of these methods, EAT has an advantage in being able to control the toner particle size and shape to fit for desired optimum cleaning, doctoring and transfer properties.
In a typical EAT, the components include pigment, wax, and styrene acrylate copolymer latex binder. These copolymers have some disadvantages as toner binders compared to polyesters, which are widely used to make conventional mechanically milled toners. Polyesters typically have better mechanical properties compared to styrene-acrylic copolymers of similar melt viscosity characteristics. This makes them more durable and resistant to filming of cartridge components. Also, polyesters have better compatibility with color pigments, resulting in a wider color gamut. Furthermore, it is possible for a polyester to obtain excellent fusing without decreasing its glass transition temperature which may hurt ship/store and other properties.
Overall, polyester binder resins are usually favored in the electrophotographic industry as toner binders, however historically they have been rarely used to make CPT, due to the synthesis method and the difficulty to make them into latex. The limited resin selection in CPT process results in limited fusing and printing properties. Low molecular weight and low molecular weight distribution polymers like polyesters typically have very poor fusing performance and durability unless the molecular weight distribution can be broadened via the addition of a cross-linking agent. This process works well for conventionally milled toners that are melt-mixed in a twin-screw extruder. However, for polyester CPT made with the EA process, it is impossible to breakdown the cross-links to allow the dissolution in a solvent such as MEK or ethyl acetate.
Unlike free-radical polymerizations for styrene-acrylic copolymers, it is extremely difficult to obtain a high molecular weight polymer, which is excellent for higher temperature fusing, in condensation polymerizations without cross-linking. In addition, for energy use efficiency in fusing, it is also required to allow the toner to reach lower temperature fusing.
Previously, a means for producing polyester CPT with a high molecular weight distribution is disclosed in pending U.S. patent application Ser. No. 11/677,838 entitled “Polyester Chemically Prepared Toners with Broad Molecular Weight Distribution” assigned to the assignee of the present invention. This enables the achievement of excellent fusibility, equivalent to what can be obtained with mechanically melt-mixed and jet-milled toners.
The properties of a dendritic polymer, especially its non-perfect analogues, star shaped and hyper-branched polymer has initiated great interest recently. Their properties, such as large number of functional groups, highly branched but soluble in organic solvents, low solution viscosity, low melt viscosity at high temperature and shear rate, and the globular three dimensional structure has been used to improve or modify the material's thermal and mechanical properties.
There are many ways to make dendritic and hyper-branched polyesters. For example, an A2+B2+A3 type of reaction using polyethylene glycol, phthalic acid and trimellitic acid monomers results in low branching and high cross-linked polymer in the condensation polymerization. This type of polyester has limited use in CPT without further treatment due to the high quantity of gel formation.
The ABx system, where x=2, 3 or more, yields a highly branched, solvent soluble, non-cross-linked polymer, but the limited sources of A and Bx monomers and its cost limit its use in this application.
To prepare a highly branched polyester for use in a CPT, the A3+B2 system is a good choice. The wide variation and availability of the tri- and di-functional monomers, A3 and B2, the topology of the polymer can provide some special properties. But this system is designed, according to Flory's theory, as a cross-linking system and will reach a gel point at a certain monomer conversion stage. It is reported that the A3:B2 ratio can, in addition with other processing methods or end capping agents, control the gel point of the reaction and even prevent the gel formation. It is a challenge to control the topology of the polymer and avoid gel formation and at the same time set up the needed properties for the polymer to be useful in a toner.